British Prime Minister Theresa May will hold an emergency meeting on July 22 to discuss Iran's seizure of a U.K.-flagged tanker three days earlier.
In a letter to the UN Security Council seen by news agencies on July 20, Britain said the tanker was in Omani territorial waters exercising its lawful right of passage when approached by Iranian forces.
"We do not seek confrontation with Iran," the letter said. "But it is unacceptable and highly escalatory to threaten shipping going about its legitimate business through internationally recognized transit corridors."
Iran has said that all 23 crew members of the seized British-flagged Stena Impero were "safe and in good health," as London condemned the seizure of the tanker, which it said was operating legally in Omani territorial waters.
"All the 23 crew members aboard the ship are safe and in good health in Bandar Abbas port," Allahmorad Afifipour, the head of the Ports and Maritime Organization of Iran in Hormozgan Province told Iranian state TV.
But Afifipour said the fate of the crew depended upon their readiness to cooperate with Iranian authorities.
"The investigation depends on the cooperation by the crew members on the vessel and also our access to the evidence required for us to look into the matter," Afifipour said.
London has denounced Iran's seizure of the oil tanker in the Gulf on July 19 as a "hostile act," and rejected Tehran's explanation that it had seized the vessel because it had been involved in an accident.
Iran's Islamic Revolutionary Guards Corps (IRGC) released a video on July 20 of its speedboats pulling alongside the oil tanker, followed by ski-mask clad troops rappelling to the ship's deck from a helicopter.
Recent actions in and around the Strait of Hormuz, one of the world's most important shipping lanes, have lifted tensions between Tehran and the West to new heights, raising fears of an armed conflict and driving up oil prices and shipping insurance rates worldwide.
British Defense Secretary Penny Mordaunt on July 20 called Iran's seizing of the tanker a "hostile act, while Foreign Secretary Jeremy Hunt said he had expressed "extreme disappointment" in a phone call with his Iranian counterpart, Mohammad Javad Zarif.
Zarif told Hunt that the ship -- which Iran claims broke international law -- must go through a legal process before it can be released, the news agency reported.
Over the past 24 hours, the British government has held two meetings of its COBRA emergency security council.
After the first meeting, Hunt said Tehran was on a "dangerous path" and threatened Iran with "considered but robust" action.
Following the second meeting, Hunt said Britain had a "desire to de-escalate" even as he stressed that the Stena Impero seizure was "in clear contravention of international law."
Iran has said the tanker was seized after it was involved in an accident with an Iranian fishing boat, something Britain and the ship operators have denied.
Meanwhile, Iran's IRGC posted a video online showing speedboats pulling alongside the tanker, with its name clearly visible. Troops in ski masks and armed with assault weapons rappelled to the ship's deck from a helicopter.
The same tactics were used by British Royal Marines on July 4 when they seized an Iranian tanker off the coast of Gibraltar on suspicions it was transporting oil to Syria in violation of EU sanctions.
That move brought condemnation from Tehran and threats from the IRGC to capture a British ship in retaliation, nearly two weeks before the Stena Impero was taken.
The Iranian tanker and crew remain impounded by Gibraltar authorities, while the Stena Impero is being held by Iranian authorities in the port of Bandar Abbas. Iranian officials say it will remain there with its 23 crew -- mostly Indian citizens, but reportedly also Russians, and others -- while the incident is investigated.
The spokesman for Iran's Guardians Council, Abbas Ali Kadkhodaei, was quoted by the semi-official Fars news agency as describing the British tanker's seizure as a legal "reciprocal action." The council works closely with Supreme Leader Ayatollah Ali Khamenei, who has final say on all state matters.
Separately, Saudi Arabia released an Iranian tanker on July 21 that Tehran says had been docked in the Jeddah port since early May for technical reasons.
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